Counter-rotating propellers have two propeller assemblies (a forward assembly and an aft assembly) that operate so that the propeller blades rotate in opposite directions to provide greater efficiency for the propeller. In order to increase or decrease the speed of the propeller, the pitch of the propeller blades can be adjusted. Changing the pitch of these blades requires communication of either an actuation power system or control signals across the interface between a non-rotating side of the propeller (e.g., the airframe or engine) and the rotating side of the propeller (e.g., the propeller assemblies). Counter-rotating propellers provide at least two challenges for changing the pitch of the blades. First, in order to transmit power and control from the non-rotating side of the propeller to the propeller assemblies, very complex, heavy and un-reliable actuation systems are required. The second challenge is providing a means for actuating the pitch of the forward propeller assembly and the aft propeller assembly concurrently, which requires crossing additional rotating boundaries. This has previously been done by using multiple pitch change actuators, typically one for each rotor. In order to overcome these and other challenges, pitch change actuation systems for counter-rotating propellers have required very complex systems with multiple components.
Counter-rotating propellers are generally rotated by a high speed gas turbine engine with the propeller rotors placed in tandem either in front (tractor) or behind the engine (pusher). In general, it is convenient to provide gear reduction to more closely match the optimal rotation speeds of the rotors and the engine. It is also convenient to use a planetary type gear train to drive the rotors in opposite directions. The planetary gearbox may be placed between the engine and first rotor or between the first and second rotor as is well known in the art. It also becomes evident that communication between the rotating propellers and the non-rotating engine case must also pass through the gearbox.